Scanning tunneling spectroscopy of SmFeAsO0.85: Possible evidence for d-wave order-parameter symmetry

We report a scanning tunneling spectroscopy investigation of polycrystalline SmFeAsO0.85 having a superconducting transition at 52 K. On large regions of the sample surface the tunneling spectra exhibited V -shaped gap structures with no coherence peaks, indicating degraded surface properties. In some regions, however, the coherence peaks were clearly observed and the V -shaped gaps could be fitted to the theory of tunneling into a d -wave superconductor, yielding gap values between 8 and 8.5 meV corresponding to the ratio 2Δ/KB TC ∼3.55-3.8, which is slightly above the BCS weak-coupling prediction. In other regions the spectra exhibited zero-bias conductance peaks consistent with a d-wave order-parameter symmetry.